Method for producing group III nitride semiconductor and template substrate

ABSTRACT

The present invention provides a method for producing a Group III nitride semiconductor. The method includes forming a groove in a surface of a growth substrate through etching; forming a buffer film on the groove-formed surface of the growth substrate through sputtering; heating, in an atmosphere containing hydrogen and ammonia, the substrate to a temperature at which a Group III nitride semiconductor of interest is grown; and epitaxially growing the Group III nitride semiconductor on side surfaces of the groove at the growth temperature. The thickness of the buffer film or the growth temperature is regulated so that the Group III nitride semiconductor is grown primarily on the side surfaces of the groove in a direction parallel to the main surface of the growth substrate. The thickness of the buffer film is regulated to be smaller than that of a buffer film which is employed for epitaxially growing the Group III nitride semiconductor on a planar growth substrate uniformly in a direction perpendicular to the growth substrate. The growth temperature is regulated to be lower than a temperature at which the Group III nitride semiconductor is epitaxially grown on a planar growth substrate uniformly in a direction perpendicular to the growth substrate. The growth temperature is preferably 1,020 to 1,100° C. The buffer film employed is an AlN film having a thickness of 150 Å or less.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for producing a Group IIInitride semiconductor product whose main surface is a non-polar plane(e.g., m-plane or a-plane) or a semi-polar plane (e.g., r-plane). Thepresent invention also relates to a template substrate including agrowth substrate and a Group III nitride semiconductor whose mainsurface is a non-polar plane or a semi-polar plane, the Group IIInitride semiconductor being formed on the growth substrate.

2. Background Art

Hitherto, Group III nitride semiconductor device have been produced bystacking a Group III nitride semiconductor on a growth substrate (e.g.,a sapphire substrate) in a c-axis direction. However, due to strain inthe crystal structure of the Group III nitride semiconductor, apiezoelectric field is generated in a c-axis direction of thesemiconductor, which may cause deterioration of device performance.When, for example, a light-emitting device is produced through theaforementioned process, internal quantum efficiency is lowered.

In recent years, in order to avoid deterioration of device performancedue to generation of a piezoelectric field, attempts have been made todevelop techniques for crystal growth of a Group III nitridesemiconductor whose main surface is a non-polar plane (e.g., a-plane orm-plane) or a semi-polar plane (e.g., r-plane). Also, attempts have beenmade to use, as a growth substrate, a GaN substrate or GaN templatesubstrate whose main surface is a non-polar plane (e.g., a-plane orm-plane) or a semi-polar plane (e.g., r-plane).

In a known crystal growth technique, a GaN substrate having an m-planeor a-plane main surface is used as a growth substrate. Such an m-planeor a-plane GaN substrate is produced by growing a thick GaN layer havinga c-plane main surface on a growth substrate (e.g., sapphire) andcutting out the thus-grown GaN layer parallel to m-plane or a-plane.

Japanese Patent Application Laid-Open (kokai) No. 2006-36561 disclosesproduction of GaN crystal or GaN template substrate having an a-plane orm-plane main surface, through the following process: stripe-patterngrooves are formed in a sapphire substrate (i.e., growth substrate)having an a-plane or m-plane main surface so that the longitudinaldirection of the grooves is a c-axis direction; an SiO₂ mask is formedon the surface of the growth substrate, one side surface of each groove,and a portion of the bottom surface of the groove; a buffer layer isformed only on the other side surface of the groove; and GaN is grown onthe side surfaces of the grooves on which the buffer layer has beenformed.

However, when an m-plane or a-plane GaN substrate is produced through aconventional method, the size of the substrate cannot be increased,since the substrate size depends on the thickness of a GaN layer. Inaddition, the conventional method fails to mass-produce such a GaNsubstrate. When a GaN layer is formed on a growth substrate, a portionof the GaN layer proximal to the substrate exhibits poor crystallinity,whereas a portion thereof distal to the substrate exhibits goodcrystallinity. Therefore, in-plane variation in crystallinity occurs inan m-plane or a-plane GaN substrate produced through cutting out of theGaN layer.

The method disclosed in Japanese Patent Application Laid-Open (kokai)No. 2006-36561 encounters difficulty in selectively forming a mask or abuffer layer, and in growing, with high reproducibility, a Group IIInitride semiconductor exhibiting good crystallinity and having ana-plane or m-plane main surface.

SUMMARY OF THE INVENTION

In view of the foregoing, an object of the present invention is toprovide a method for producing a Group III nitride semiconductorproduct, which method realizes formation, with high reproducibility, ofa Group III nitride semiconductor product whose main surface is anon-polar plane (e.g., a-plane or m-plane) or a semi-polar plane (e.g.,r-plane). Another object of the present invention is to provide atemplate substrate including a growth substrate and a Group III nitridesemiconductor whose main surface is a non-polar plane or a semi-polarplane, the semiconductor being formed on the growth substrate.

In a first aspect of the present invention, there is provided a methodfor producing a Group III nitride semiconductor product, comprising thesteps of:

forming a groove in a surface of a growth substrate through etching;

heating, in an atmosphere containing hydrogen and ammonia, the growthsubstrate having the thus-formed groove to a temperature at which aGroup III nitride semiconductor of interest is grown; and

epitaxially growing the Group III nitride semiconductor on side surfacesof the groove at the growth temperature, wherein the growth temperatureis regulated so that the Group III nitride semiconductor is grownprimarily on the side surfaces of the groove in a direction parallel tothe main surface of the growth substrate.

A Group III nitride semiconductor can be grown on the side surfaces ofthe groove primarily in a direction parallel to the main surface of thegrowth substrate by, for example, regulating the growth temperature tobe lower than a temperature at which the Group III nitride semiconductoris grown on a planar growth substrate in a direction perpendicular tothe substrate. In general, a Group III nitride semiconductor is grown ina c-axis direction at a temperature higher than 1,100° C. Therefore,when the growth temperature is 1,100° C. or lower, a Group III nitridesemiconductor can be grown on the side surfaces of the groove primarilyin a c-axis direction. The growth temperature is preferably 1,020° C. orhigher. This is because, when the growth temperature is lower than1,020° C., the resultant Group III nitride semiconductor exhibits poorcrystallinity. Thus, the growth temperature of the Group III nitridesemiconductor is preferably regulated to 1,020 to 1,100° C. The growthtemperature is more preferably 1,020 to 1,060° C., from the viewpointsof better crystallinity and surface flatness of a Group III nitridesemiconductor. The growth temperature is much more preferably 1,030 to1,050° C.

A second aspect of the present invention is drawn to a specificembodiment of the production method according to the first aspect, whichfurther comprises, between the step of forming the groove and the stepof heating the growth substrate, the step of forming a buffer film onthe surface of the growth substrate through sputtering, wherein thethickness of the buffer film is regulated so that the Group III nitridesemiconductor is grown primarily on the side surfaces of the groove in adirection parallel to the main surface of the growth substrate.

That is, in the second aspect of the present invention, a buffer film isformed on the growth substrate, and the first aspect is carried out.

For formation of the buffer film, magnetron sputtering is preferablyemployed among sputtering techniques. No particular limitation isimposed on the material of the buffer film, so long as a Group IIInitride semiconductor can be grown primarily on the side surfaces of thegroove in a direction parallel to the main surface of the growthsubstrate. The material of the buffer film may be, for example, GaN,AlN, AlGaN, or AlGaInN. Particularly, AlN is preferably employed, fromthe viewpoint of lattice matching with sapphire.

A Group III nitride semiconductor can be epitaxially grown on the sidesurfaces of the groove primarily in a direction parallel to the mainsurface of the growth substrate by, for example, regulating thethickness of the buffer film to be smaller than that of a buffer filmwhich is provided on a planar growth substrate when the Group IIInitride semiconductor is grown, via the buffer film, on the planargrowth substrate uniformly in a direction perpendicular to the planargrowth substrate, and regulating the growth temperature to be lower thana temperature at which the Group III nitride semiconductor is grown, viathe buffer film, on the planar growth substrate uniformly in a directionperpendicular to the main surface of the planar growth substrate. Whenan AlN buffer film is formed on the flat main surface of a sapphiresubstrate, and a Group III nitride semiconductor is grown perpendicularto the main surface of the buffer film in a c-axis direction, generally,the thickness of the buffer film is regulated to at least 150 to 200 Å.In contrast, in the present invention, the thickness of the buffer filmis regulated to 150 Å or less, in order to increase the rate of growthof a Group III nitride semiconductor in a direction perpendicular to theside surfaces of the groove of the growth substrate to be higher thanthat of growth of the semiconductor in a direction perpendicular to themain surface of the substrate. Thus, the thickness of the buffer film ispreferably 150 Å or less. From the viewpoint of good surface flatness ofthe thus-grown Group III nitride semiconductor, the thickness of thebuffer film is preferably 55 Å or more. Therefore, the thickness of thebuffer film is preferably 55 Å to 150 Å. More preferably, the thicknessis regulated to 55 to 125 Å, from the viewpoints of better crystallinityand surface flatness of a Group III nitride semiconductor. The thicknessis much more preferably 75 to 125 Å. The buffer film having such athickness is preferably formed from AlN.

In general, a Group III nitride semiconductor is grown, via a bufferfilm, on a planar sapphire substrate in a c-axis direction at atemperature higher than 1,100° C. Therefore, when the growth temperatureis 1,100° C. or lower, a Group III nitride semiconductor can be grown onthe side surfaces of the groove primarily in a direction perpendicularto the growth substrate. The growth temperature is preferably 1,020° C.or higher. This is because, when the growth temperature is lower than1,020° C., the resultant Group III nitride semiconductor exhibits poorcrystallinity. Thus, the growth temperature of the Group III nitridesemiconductor is preferably regulated to 1,020 to 1,100° C. The growthtemperature is more preferably 1,020 to 1,060° C., from the viewpointsof better crystallinity and surface flatness of a Group III nitridesemiconductor. The growth temperature is much more preferably 1,030 to1,050° C. The thickness of the buffer film must be smaller than athickness at which the Group III nitride semiconductor is grown in adirection perpendicular to the main surface of the growth substrate, andmust be a thickness at which crystallinity is not impaired. Mostpreferably, the buffer film is formed from AlN, and the thicknessthereof is regulated to 150 Å or less.

In the present invention, by virtue of the presence of the buffer film,a Group III nitride semiconductor is grown not on the flat surface ofthe growth substrate, but on the side surfaces of the groove in alateral direction.

The following description is common to the first and second aspects ofthe present invention.

As used herein, “Group III nitride semiconductor” encompasses asemiconductor represented by the formula Al_(x)Ga_(y)In_(z)N (x+y+z=1,0x, y, z≦1); such a semiconductor doped with an impurity so as toattain, for example, an n-type conduction or a p-type conduction; aswell as such a semiconductor in which a portion of Al, Ga, or In issubstituted by another Group 13 element (i.e., B or Tl), or a portion ofN is substituted by another Group 15 element (i.e., P, As, Sb, or Bi).Specific examples of the Group III nitride semiconductor include GaN,AlN, InN, InGaN, AlGaN, AlInN, and AlGaInN. Si is employed as an n-typeimpurity, and Mg is employed as a p-type impurity.

As used herein, “side surfaces of a groove” refers to, among surfacesexposed through formation of a groove in the growth substrate, surfaceswhich are not parallel to the main surface of the growth substrate.

The growth substrate may be formed of a hexagonal material such assapphire, SiC, Si, GaAs, ZnO, or spinel. However, a sapphire substrateis preferably employed, from the viewpoints of, for example, easyavailability and lattice matching with a Group III nitridesemiconductor.

Grooves formed in a surface of the growth substrate may be arranged inany pattern as viewed from above; for example, a stripe pattern, a gridpattern, or a hexagonal, triangular, or circular dot pattern. The crosssection of each groove as viewed in a vertical direction of the growthsubstrate may have any form (e.g., a rectangular form, a trapezoidalform, or a wedge-like form), which may vary with the crystal orientationof the main surface of a Group III nitride semiconductor of interest.

The growth temperature is preferably lower than a temperature at whichthe Group III nitride semiconductor is epitaxially grown on a planargrowth substrate uniformly in a direction perpendicular to the growthsubstrate.

The growth substrate employed is preferably a sapphire substrate. Insuch a case, preferably, the angle between c-plane or a-plane ofsapphire and side surfaces of a groove on which a Group III nitridesemiconductor is grown is reduced to a minimum possible level. This isbecause, the smaller the aforementioned angle, the more easily a GroupIII nitride semiconductor is grown in a c-axis direction. In otherwords, a Group III nitride semiconductor is difficult to grow on sidesurfaces which form a large angle with c-plane or a-plane of sapphire.Most preferably, side surfaces of a groove on which a Group III nitridesemiconductor is grown assume a c-plane or a-plane.

Preferably, at least one of side surfaces of a groove assumes a c-planeof sapphire, and a Group III nitride semiconductor is epitaxially grownon the side surface(s) which assumes (assume) a c-plane of sapphire. Thegrowth substrate employed may be a sapphire substrate having an a-planemain surface. Alternatively, the growth substrate may be a sapphiresubstrate having an m-plane main surface. In such a case, preferably,grooves are formed in a stripe pattern so that the longitudinal sidesurfaces thereof assume a c-plane of sapphire.

Also, preferably, at least one of side surfaces of a groove formed inthe growth substrate is a-plane of sapphire, and a Group III nitridesemiconductor is epitaxially grown primarily on the side surface(s)which assumes (assume) an a-plane of sapphire. In this case, the mainsurface of the growth substrate is preferably c-plane. Preferably,grooves are formed in a stripe pattern so that the longitudinal sidesurfaces thereof are a-plane of sapphire.

For formation of grooves, dry etching (e.g., ICP etching) may beemployed.

In a third aspect of the present invention, there is provided a templatesubstrate comprising:

a sapphire substrate having an a-plane main surface and having a grooveformed in the surface; and

a Group III nitride semiconductor layer formed on the surface of thesapphire substrate, wherein at least one surface of the groove assumes ac-plane of sapphire, and the Group III nitride semiconductor layer hasan m-plane main surface.

According to the third aspect of the present invention, the Group IIInitride semiconductor layer is grown in a direction perpendicular to theside surface(s) of the groove (i.e., c-plane of sapphire); i.e., thelayer is grown in an c-axis direction. Since the main surface of thesapphire substrate is a-plane, the main surface of the Group III nitridesemiconductor layer parallel to the growth direction assumes an m-plane.

In a fourth aspect of the present invention, there is provided atemplate substrate comprising:

a sapphire substrate having an m-plane main surface and having a grooveformed in the surface; and

a Group III nitride semiconductor layer formed on the surface of thesapphire substrate, wherein at least one surface of the groove assumes ac-plane of sapphire, and the Group III nitride semiconductor layer hasan a-plane main surface.

According to the fourth aspect of the present invention, the Group IIInitride semiconductor layer is grown in a direction perpendicular to theside surface(s) of the groove (i.e., c-plane of sapphire); i.e., thelayer is grown in an c-axis direction. Since the main surface of thesapphire substrate assumes an m-plane, the main surface of the Group IIInitride semiconductor layer parallel to the growth direction is a-plane.

In a fifth aspect of the present invention, there is provided a templatesubstrate comprising:

a sapphire substrate having a c-plane main surface and having a grooveformed in the surface; and

a Group III nitride semiconductor layer formed on the surface of thesapphire substrate, wherein at least one surface of the groove isa-plane of sapphire, and the Group III nitride semiconductor layer hasan a-plane main surface.

According to the fifth aspect of the present invention, the Group IIInitride semiconductor layer is grown in a direction perpendicular to theside surface(s) of the groove (i.e., a-plane of sapphire); i.e., thelayer is grown in an c-axis direction. Since the main surface of thesapphire substrate assumes a c-plane, the main surface of the Group IIInitride semiconductor layer parallel to the growth direction is a-plane.

A sixth aspect of the present invention is drawn to a specificembodiment of the template substrate according to the third, fourth, orfifth aspect, which further comprises, between the sapphire substrateand the Group III nitride semiconductor layer, a buffer film formed soas to cover the surface of the sapphire substrate, the side surfaces ofthe groove, and the bottom surface of the groove, wherein the Group IIInitride semiconductor layer is formed, via the buffer film, on thesurface of the sapphire substrate.

In the third or fourth aspect of the present invention, preferably, thegroove is formed in a stripe pattern such that the groove has alongitudinal side surface which assumes a c-plane of sapphire.

In the fifth aspect of the present invention, preferably, the groove isformed in a stripe pattern such that the groove has a longitudinal sidesurface which is a-plane of sapphire.

Preferred modes described in the first and second aspects of the presentinvention may be applied to the third to sixth aspects of the presentinvention. Preferred modes (regarding the buffer film) described in thesecond aspect may be applied to the sixth aspect.

According to the first aspect of the present invention, a Group IIInitride semiconductor can be epitaxially grown on the side surfaces of agroove formed in a growth substrate in a direction parallel to the mainsurface of the growth substrate. Therefore, a high-quality Group IIInitride semiconductor whose main surface is a non-polar plane or asemi-polar plane can be formed. Since the method according to the firstaspect does not include a step of restoring etching damage throughthermal treatment or a similar treatment of the growth substrate, theproduction process is simplified, and excellent mass productivity isachieved. The crystal orientation of the main surface of a Group IIInitride semiconductor formed depends on, for example, the crystalstructure or lattice constant of the growth substrate, the crystalorientation of the main surface of the growth substrate, or the crystalorientation of the side surfaces of the groove. In the case where, forexample, the growth substrate is a sapphire substrate, when the mainsurface of the substrate is a-plane, and the side surfaces of the grooveassume a c-plane, a Group III nitride semiconductor having an m-planemain surface can be produced, whereas when the main surface of thesubstrate assumes an m-plane, and the side surfaces of the groove assumea c-plane, a Group III nitride semiconductor having an a-plane mainsurface can be produced.

According to the second aspect of the present invention, a buffer filmis provided through sputtering without provision of a mask used in awell-known ELO technique, and a Group III nitride semiconductor isepitaxially grown on the side surfaces of a groove formed in a growthsubstrate in a direction parallel to the main surface of the growthsubstrate. Therefore, a high-quality Group III nitride semiconductorwhose main surface is a non-polar plane or a semi-polar plane can beformed. Since the buffer film is formed through sputtering, the filmexhibits reliable characteristics. Thus, a Group III nitridesemiconductor whose main surface is a non-polar plane or a semi-polarplane can be formed with high reproducibility, and excellent massproductivity is achieved. Since the method according to the secondaspect does not include a step of restoring etching damage throughthermal treatment or a similar treatment of the growth substrate, aGroup III nitride semiconductor whose main surface is a non-polar planeor a semi-polar plane can be formed through a simple process. Thecrystal orientation of the main surface of a Group III nitridesemiconductor formed depends on, for example, the crystal structure orlattice constant of the growth substrate, the crystal orientation of themain surface of the growth substrate, or the crystal orientation of theside surfaces of the groove. In the case where, for example, the growthsubstrate is a sapphire substrate, when the main surface of thesubstrate is a-plane, and the side surfaces of the groove assume ac-plane, a Group III nitride semiconductor having an m-plane mainsurface can be produced, whereas when the main surface of the substrateassumes an m-plane, and the side surfaces of the groove assume ac-plane, a Group III nitride semiconductor having an a-plane mainsurface can be produced.

When the growth temperature is regulated to be lower than a temperatureat which the Group III nitride semiconductor is epitaxially grown on aplanar growth substrate uniformly in a direction perpendicular to thegrowth substrate, the Group III nitride semiconductor can be epitaxiallygrown on the side surfaces of the groove formed in the growth substrateprimarily in a direction parallel to the main surface of the growthsubstrate. That is, the growth rate in a direction perpendicular to theside surfaces of the groove can be controlled to be higher than that ina direction perpendicular to the main surface of the growth substrate.

When the thickness of the buffer film is regulated to be smaller thanthat of a buffer film which is employed for epitaxially growing theGroup III nitride semiconductor on a planar growth substrate uniformlyin a direction perpendicular to the growth substrate, and when thegrowth temperature is regulated to be lower than a temperature at whichthe Group III nitride semiconductor is epitaxially grown on a planargrowth substrate uniformly in a direction perpendicular to the growthsubstrate, the Group III nitride semiconductor can be epitaxially grownon the side surfaces of the groove formed in the growth substrateprimarily in a direction parallel to the main surface of the growthsubstrate. That is, the growth rate in a direction perpendicular to theside surfaces of the groove can be controlled to be higher than that ina direction perpendicular to the main surface of the growth substrate.

The buffer film may be formed of Al_(x)Ga_(1-x)N or AlN. When an AlNbuffer film having a thickness of 150 Å or less is provided, there canbe produced a Group III nitride semiconductor whose main surface is anon-polar plane or a semi-polar plane and which exhibits furtherexcellent crystallinity and surface flatness.

Since the growth substrate can be formed of sapphire, there can beproduced, at low cost, a Group III nitride semiconductor whose mainsurface is a non-polar plane or semi-polar plane and has a large area.

Since a Group III nitride semiconductor crystal is likely to grow onc-plane or a-plane of sapphire, when the side surfaces of the grooveassume a c-plane or a-plane, a Group III nitride semiconductor whosemain surface is a non-polar plane or a semi-polar plane and whichexhibits good crystallinity can be effectively produced.

When the growth substrate is a sapphire substrate having an a-plane mainsurface, and a Group III nitride semiconductor is epitaxially grown onthe c-plane side surfaces of the groove in a c-axis direction, thethus-produced Group III nitride semiconductor has an m-plane mainsurface. In contrast, when the growth substrate is a sapphire substratehaving an m-plane main surface, and a Group III nitride semiconductor isepitaxially grown on the c-plane side surfaces of the groove in a c-axisdirection, a Group III nitride semiconductor having an a-plane mainsurface can be produced.

When a stripe-pattern groove whose longitudinal side surfaces assume ac-plane of sapphire is provided, there can be produced a Group IIInitride semiconductor whose main surface is a non-polar plane or asemi-polar plane and which exhibits excellent crystallinity and surfaceflatness.

The growth temperature of the Group III nitride semiconductor ispreferably 1,020 to 1,100° C. When the growth temperature falls withinsuch a range, a Group III nitride semiconductor can be grown on the sidesurfaces of the groove predominantly in a c-axis direction, and thecrystallinity and surface flatness of the produced Group III nitridesemiconductor can be enhanced.

When the growth substrate is a sapphire substrate having a c-plane mainsurface, and a Group III nitride semiconductor is epitaxially grown onthe a-plane side surfaces of the groove in a c-axis direction, a GroupIII nitride semiconductor having an a-plane main surface can beproduced.

When a stripe-pattern groove whose longitudinal side surfaces area-plane of sapphire is provided, there can be produced a Group IIInitride semiconductor whose main surface is a non-polar plane or asemi-polar plane and which exhibits excellent crystallinity and surfaceflatness.

The method of the present invention may employ dry etching. The growthsubstrate can be effectively etched by means of ICP etching.

In the template substrate according to any of the third to sixth aspectsof the present invention, the Group III nitride semiconductor layerexhibits excellent crystallinity and surface flatness. When asemiconductor device is produced by stacking a Group III nitridesemiconductor layer on the template substrate, the resultantsemiconductor device is not affected by a piezoelectric field.

BRIEF DESCRIPTION OF THE DRAWINGS

Various other objects, features, and many of the attendant advantages ofthe present invention will be readily appreciated as the same becomesbetter understood with reference to the following detailed descriptionof the preferred embodiments when considered in connection with theaccompanying drawings, in which:

FIGS. 1A to 1C are sketches showing a process for producing a GaNtemplate substrate according to Embodiment 1;

FIG. 2 shows the results of X-ray diffractometry of surfaces of GaNcrystals 13;

FIG. 3 is a graph showing the relationship between hydrogen flow rate ina heating step and X-ray rocking curve half width;

FIG. 4 is photographs showing surfaces of GaN crystals 13;

FIGS. 5A to 5D are sketches showing a process for producing a GaNtemplate substrate according to Embodiment 2;

FIG. 6 shows the results of X-ray diffractometry of surfaces of GaNcrystals 13;

FIG. 7 is a graph showing the relationship between sputtering time forformation of an AlN film 12 and X-ray rocking curve half width;

FIG. 8 is photographs showing surfaces of GaN crystals 13; and

FIG. 9 is a graph showing the relationship between sputtering time forformation of an AlN film 12 and X-ray rocking curve half width whenusing 3-inches substrate.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Specific embodiments of the present invention will next be describedwith reference to the drawings. However, the present invention is notlimited to the embodiments.

Embodiment 1

Embodiment 1 corresponds to a method for producing a GaN templatesubstrate having an m-plane main surface. Steps of the production methodwill now be described with reference to FIGS. 1A to 1C.

(Groove Formation Step)

Firstly, a surface 10 a of a sapphire substrate 10 having an a-planemain surface (corresponding to the growth substrate of the presentinvention) is subjected to ICP etching by use of a mask, to thereby formstripe-pattern grooves 11 whose longitudinal direction is parallel tothe m-axis direction of the sapphire substrate 10 (FIG. 1A). The crosssection of each groove 11 parallel to the c-axis assumes a rectangularform. C-plane surfaces of the sapphire substrate are exposed at the sidesurfaces 11 a of each groove 11, and an a-plane surface of the sapphiresubstrate is exposed at the bottom surface 11 b of the groove 11.

In general, after formation of the grooves 11, the sapphire substrate 10is heated to 1,000° C. or higher for restoring damage to the sapphiresubstrate 10 caused by ICP etching. In contrast, in Embodiment 1, thesubstrate 10 is not subjected to such a thermal treatment for restoringdamage thereto, and thus the substrate 10 remains to have damage to theside surfaces 11 a and the bottom surfaces 11 b of the grooves 11 causedby ICP etching.

(Heating Step)

Subsequently, the sapphire substrate 10 is placed into an MOCVDapparatus and heated to a growth temperature in an atmosphere containinghydrogen and ammonia.

(Crystal Growth Step)

Then, TMG (trimethylgallium) is introduced into the MOCVD apparatus, andGaN crystal 13 is epitaxially grown on the side surfaces 11 a of eachgroove 11. GaN crystal 13 is grown so that the c-axis direction of thesapphire substrate 10 coincides with the c-axis direction of the GaNcrystal 13. Regarding the polarity of the c-axis direction of the GaNcrystal 13, the -c direction of GaN corresponds to the direction fromthe side surfaces 11 a of the groove 11 toward the inside (center) ofthe groove 11. That is, growth of GaN proceeds in a-c-axis direction(i.e., in a direction perpendicular to the side surfaces 11 a), and thegrowth surface thereof assumes a-c-plane.

In this case, the temperature for the growth of GaN crystal 13 isregulated so that GaN crystal 13 is not grown on the surface 10 a of thesapphire substrate 10 or the bottom surfaces 11 b of the grooves 11, andthat growth of GaN crystal 13 on the side surfaces 11 a of the grooves11 proceeds predominantly in a c-axis direction. For example, the growthtemperature of GaN crystal 13 is regulated to be lower than atemperature at which, generally, GaN is epitaxially grown on a flatgrowth substrate in a direction perpendicular to the substrate. Thetemperature at which, generally, GaN is epitaxially grown on a flatgrowth substrate in a direction perpendicular to the substrate is higherthan 1,100° C. Therefore, when the GaN growth temperature is regulatedto 1,100° C. or lower, crystal growth can be controlled so that GaNcrystal 13 is not grown on the surface 10 a of the sapphire substrate 10or the bottom surfaces 11 b of the grooves 11, and that growth of GaNcrystal 13 on the side surfaces 11 a of the grooves 11 proceedspredominantly in a c-axis direction.

When GaN crystal 13 is grown as described above, growth of GaN crystal13 proceeds rapidly toward the inside of each groove 11 in a c-axisdirection (-c direction); i.e., in a direction horizontal to thesapphire substrate 10, and also proceeds gradually in a directionperpendicular to the sapphire substrate 10 (FIG. 1B). When crystalgrowth proceeds further, the grooves 11 are filled with GaN, and thesurface 10 a of the sapphire substrate 10 is gradually covered with GaNthrough crystal growth in directions horizontal to the sapphiresubstrate 10 (i.e., both -c direction and +c direction). Finally, a flatGaN crystal layer 13 is formed on the sapphire substrate 10 (FIG. 1C).The thus-formed GaN crystal 13 has an m-plane main surface, since theside surfaces 11 a of the grooves 11 of the sapphire substrate 10 assumea c-plane. This is attributed to, for example, the difference in latticeconstant between GaN and sapphire.

As described above, the GaN template substrate production method ofEmbodiment 1 produces GaN crystal 13 having an m-plane main surface andexhibiting good crystallinity and surface flatness. Conceivable reasonstherefor will be described below. Conceivably, damage to the sapphiresubstrate caused by ICP etching prevents growth of GaN crystal having ac-plane main surface on the sapphire substrate having an a-plane mainsurface. Also, conceivably, when the growth temperature of GaN crystal13 is regulated to an appropriate value, GaN crystal 13 is not grown onthe surface 10 a of the sapphire substrate 10 or the bottom surfaces 11b of the grooves 11 (i.e., GaN crystal 13 is epitaxially grown only onthe side surfaces 11 a of the grooves 11), and growth of GaN crystal 13proceeds predominantly in a c-axis direction; i.e., in a directionhorizontal to the sapphire substrate 10. In this case, the surface ofthe GaN crystal 13 parallel to the sapphire substrate 10 assumes anm-plane, which is attributed to, for example, lattice matching betweensapphire and GaN. Also, conceivably, an AlN film is formed throughnitridation of sapphire in the heating step, and the AlN film serves asa buffer for facilitating epitaxial growth of GaN on the side surfaces11 a of the grooves 11. For these reasons, the GaN crystal 13 does notcontain a crystal having a c-plane surface parallel to the sapphiresubstrate 10, and has an m-plane main surface. Since growth of GaNcrystal 13 proceeds predominantly in a direction horizontal to thesapphire substrate 10, the GaN crystal 13 can rapidly cover the surface10 a of the sapphire substrate 10, and the surface 13 a of the GaNcrystal 13 becomes flat.

Dependencies, on various factors, of the crystallinity of GaN crystal 13and the flatness of the surface 13 a were investigated by the followingexperiments.

FIG. 2 shows the results of X-ray diffractometry of a surface 13 a ofGaN crystal 13 grown at 1,040° C. on a sapphire substrate 10 having ana-plane main surface (diameter: 3 inches) and having grooves 11 (width:1.5 μm, depth: 0.7 μm, interval: 1.5 μm). For comparison, FIG. 2 alsoshows the results corresponding to the case where a sapphire substrate10 having no groove was employed. In this experiment, hydrogen flow ratein the heating step was regulated to a standard rate (i.e., hydrogenflow rate in the case where GaN crystal is grown on a flat sapphiresubstrate uniformly in a vertical direction), or regulated to be thriceor four times the standard rate. As shown in FIG. 2, when the sapphiresubstrate 10 having no groove 11 was employed, peaks attributed to(0002) plane and (0004) plane (both assume a c-plane) were observed, butno peak attributed to m-plane was observed; i.e., GaN crystal having anm-plane main surface was not produced. In contrast, when GaN crystal 13was formed through the method described in Embodiment 1, regardless ofhydrogen flow rate, peaks attributed to (10-10) plane and (20-20) plane(both assume an m-plane) were observed, but no peak attributed toc-plane or a-plane was observed. These data indicate that the GaNcrystal 13 formed through the method described in Embodiment 1 has anm-plane main surface and exhibits high crystallinity.

FIG. 3 is a graph showing the relationship between hydrogen flow rate inthe heating step and X-ray rocking curve half width in the c-plane orm-plane of GaN crystal 13. The sizes of the sapphire substrate 10 andthe grooves 11, and the growth temperature of GaN crystal 13 were thesame as those in the experiment whose results are shown in FIG. 2.Hydrogen flow rate in the heating step was varied in the same manner asin the case of the experiment whose results are shown in FIG. 2. Forobtaining data corresponding to m-plane, the X-ray rocking curve halfwidths in c-axis and a-axis directions of m-plane of a sample weredetermined by rotating the sample about both the a-axis and the c-axis.In the plotted data of X-ray rocking curve half width shown in FIG. 3,squares, triangles, and circles correspond to c-plane, c-axis directionof m-plane, and a-axis direction of m-plane, respectively.

As shown in FIG. 3, the X-ray rocking curve half width in a c-axisdirection of m-plane was about 1,000 arcsec (hydrogen flow rate: thestandard rate) or about 1,500 arcsec (hydrogen flow rate: thrice or fourtimes the standard rate). Thus, the crystal orientation in a c-axisdirection of m-plane was high when hydrogen flow rate was the standardrate. The X-ray rocking curve half width in an a-axis direction ofm-plane was about 500 arcsec at any of the aforementioned hydrogen flowrates; i.e., the crystal orientation in an a-axis direction of m-planewas high, regardless of hydrogen flow rate. The X-ray rocking curve halfwidth in the c-plane was about 1,500 arcsec (hydrogen flow rate: thestandard rate) or about 2,500 arcsec (hydrogen flow rate: thrice or fourtimes the standard rate). Thus, the crystal orientation in the c-planewas high when hydrogen flow rate was the standard rate.

As is clear from the data shown in FIGS. 2 and 3, the GaN crystal 13exhibits the most excellent crystallinity when hydrogen flow rate isadjusted to the standard rate. In this case, the hydrogen content of ahydrogen-ammonia mixture atmosphere is about 50%.

FIG. 4 shows photographs of the surfaces of GaN crystals 13corresponding to the cases where hydrogen flow rate was the standardrate, thrice the standard rate, and four times the standard rate. Forcomparison, FIG. 4 also shows a photograph of the surface of GaN crystal13 grown on a sapphire substrate 10 having no groove 11. The sizes ofthe sapphire substrate 10 and the grooves 11, and the growth temperatureof GaN crystal 13 were the same as those in the experiment whose resultsare shown in FIG. 2 or 3. As is clear from FIG. 4, when hydrogen flowrate is the standard rate, high surface flatness is achieved, ascompared with the case where hydrogen flow rate is thrice or four timesthe standard rate. In contrast, when GaN crystal is grown on thesapphire substrate 10 having no groove 11, numerous hexagonalmicroirregularities are formed on the surface of the GaN crystal; i.e.,the GaN crystal exhibits poor surface flatness.

GaN crystals 13 were grown at different growth temperatures, and thesurfaces 13 a of the thus-grown GaN crystals 13 were observed. The GaNcrystals grown at 1,020 to 1,060° C. exhibited high surface flatness.Particularly, the GaN crystals grown at 1,030 to 1,050° C. exhibitedhigher surface flatness. The GaN crystal grown at 1,040° C. exhibitedthe most excellent surface flatness.

In Embodiment 1, a sapphire substrate is employed as the growthsubstrate. However, the growth substrate may be, instead of a sapphiresubstrate, a substrate formed of a hexagonal material such as SiC, Si,GaAs, ZnO, or spinel. In Embodiment 1, ICP etching is employed forforming grooves in the growth substrate. However, another dry etchingtechnique may be employed.

Embodiment 1 corresponds to a method for producing a GaN templatesubstrate. However, the present invention is not limited to GaN, but isapplicable to a Group III nitride semiconductor such as AlN, AlGaN,InGaN, AlInN, or AlGaInN. The present invention is not limited to theproduction of such a Group III nitride semiconductor described inEmbodiment 1 (i.e., Group III nitride semiconductor having an m-planemain surface), and can produce a Group III nitride semiconductor whosemain surface is any non-polar plane or semi-polar plane, inconsideration of the crystal orientation of the main surface of thegrowth substrate employed, the crystal orientation of the side surfacesof grooves formed in the growth substrate, and the lattice constant ofthe growth substrate. For example, when a sapphire substrate having anm-plane main surface is employed, and grooves are formed so that theside surfaces thereof assume a c-plane, a Group III nitridesemiconductor having an a-plane main surface can be produced. Also, whena sapphire substrate having a c-plane main surface is employed, andgrooves are formed so that the side surfaces thereof are a-plane, aGroup III nitride semiconductor having an a-plane main surface can beproduced.

In Embodiment 1, grooves are formed so that they are arranged in astripe pattern as viewed from above. However, grooves may be formed inany pattern; for example, a grid pattern or a dot pattern. Since a GroupIII nitride semiconductor has polarity particularly in a c-axisdirection, when the side surfaces of a groove have different crystalorientations, GaN crystal having different polarity directions is grown.Therefore, when grooves are formed in such a pattern that the sidesurfaces of each groove have many different crystal orientations, aGroup III nitride semiconductor crystal having many different polaritydirections is produced, which is not preferred. From such a viewpoint,stripe-pattern grooves are more advantageous than grooves formed inanother pattern, since each stripe-pattern groove has only two sidesurfaces, and a Group III nitride semiconductor grown on the sidesurfaces exhibits polarity in only two directions. Stripe-patterngrooves are also advantageous in that the side surfaces of each groovehave a large area, and thus the crystallinity and surface flatness of aGroup III nitride semiconductor grown on the side surfaces are higherthan those of a Group III nitride semiconductor grown on the sidesurfaces of grooves formed in another pattern.

In order to avoid growth of a crystal having different polaritydirections, some of side surfaces of grooves may be covered with, forexample, a mask, so that a Group III nitride semiconductor crystal isnot grown on the thus-covered side surfaces. For example, when groovesare formed in a stripe pattern, and one of two side surfaces of eachgroove is covered with a mask so that a Group III nitride semiconductoris grown only on the other side surface, the resultant Group III nitridesemiconductor exhibits polarity only in one direction; i.e., thesemiconductor exhibits good quality.

Alternatively, in order to avoid growth of a crystal having differentpolarity directions, side surfaces of grooves may be inclined so thatthe side surfaces exhibits a crystal orientation in which crystal growthis less likely to occur. When, for example, a sapphire substrate isemployed as a growth substrate, the smaller the angle between the sidesurfaces of grooves formed in the substrate and the c-plane or a-planeof sapphire, the more likely a crystal is grown on the side surfaces. Inthis case, when the side surfaces of the grooves assume a c-plane ora-plane, a crystal is most likely to grow on the side surfaces.Therefore, when, for example, side surfaces of grooves on which a GroupIII nitride semiconductor crystal is grown assume a c-plane, and theother groove side surfaces on which no crystal growth is desired areinclined with respect to c-plane, growth of a crystal having differentpolarity directions can be avoided.

Embodiment 2

Similar to the case of Embodiment 1, Embodiment 2 corresponds to amethod for producing a GaN template substrate having an m-plane mainsurface. Steps of the production method will next be described withreference to FIGS. 5A to 5D.

(Groove Formation Step)

Firstly, a surface 10 a of a sapphire substrate 10 having an a-planemain surface (corresponding to the growth substrate of the presentinvention) is subjected to ICP etching by use of a mask, to thereby formstripe-pattern grooves 11 whose longitudinal direction is parallel tothe m-axis direction of the sapphire substrate 10 (FIG. 5A). The crosssection of each groove 11 parallel to the c-axis assumes a rectangularform. C-plane surfaces of the sapphire substrate are exposed at the sidesurfaces 11 a of each groove 11, and an a-plane surface of the sapphiresubstrate is exposed at the bottom surface 11 b of the groove 11.

(Buffer Film Formation Step)

Subsequently, the sapphire substrate 10 having the thus-formed groovesis introduced into a reactive magnetron sputtering apparatus, and an AlNfilm 12 (corresponding to the buffer film of the present invention) isformed at 500° C. (FIG. 5B). In this case, the AlN film 12 is formed notonly on the surface 10 a of the sapphire substrate 10, but also on theside surfaces 11 a and the bottom surfaces 11 b of the grooves 11. Itwas observed that the thickness of a portion of the AlN film 12 formedon the side surfaces 11 a of the grooves 11 is smaller than that of aportion of the AlN film 12 formed on the surface 10 a of the sapphiresubstrate 10 and on the bottom surfaces 11 b of the grooves 11. As usedherein, “the thickness of the AlN film 12” refers to the thickness of aportion of the AlN film 12 formed on the surface 10 a of the sapphiresubstrate 10.

In general, before formation of the AlN film 12, the sapphire substrate10 is heated to 1,000° C. or higher for restoring damage to the sapphiresubstrate 10 caused by ICP etching. In contrast, in Embodiment 2, thesubstrate 10 is not subjected to such a thermal treatment for restoringdamage thereto, and the AlN film 12 is formed, with the substrate 10having damage to the side surfaces 11 a and the bottom surfaces 11 b ofthe grooves 11 caused by ICP etching.

(Heating Step)

Subsequently, the sapphire substrate 10 having the thus-formed AlN film12 is placed into an MOCVD apparatus and heated to a growth temperaturein an atmosphere containing hydrogen and ammonia.

(Crystal Growth Step)

Then, TMG (trimethylgallium) is introduced into the MOCVD apparatus, andGaN crystal 13 is epitaxially grown on the side surfaces 11 a of eachgroove 11. GaN crystal 13 is grown so that the c-axis direction of thesapphire substrate 10 coincides with the c-axis direction of the GaNcrystal 13. Regarding the polarity of the c-axis direction of the GaNcrystal 13, -c direction corresponds to the direction from the sidesurfaces 11 a of the groove 11 toward the inside (center) of the groove11. That is, growth of GaN proceeds in a-c-axis direction (i.e., in adirection perpendicular to the side surfaces 11 a), and the growthsurface thereof assumes a-c-plane.

In this case, the thickness of the AlN film 12 and the growthtemperature of GaN crystal 13 are regulated so that GaN crystal 13 isnot grown on the surface 10 a of the sapphire substrate 10 or the bottomsurfaces 11 b of the grooves 11, and that growth of GaN crystal 13 onthe side surfaces 11 a of the grooves 11 proceeds predominantly in ac-axis direction. For example, the thickness of the AlN film 12 isregulated to be smaller than the minimum thickness of an AlN film whichis provided between a sapphire substrate and GaN when GaN is epitaxiallygrown evenly in a c-axis direction of GaN (i.e., in a directionperpendicular to the main surface of the sapphire substrate). Also, thegrowth temperature of GaN crystal 13 is regulated to be lower than atemperature at which, generally, GaN is epitaxially grown on a growthsubstrate in a c-axis direction (i.e., in a direction perpendicular tothe main surface of the substrate). Generally, an AlN film having such aminimum thickness (150 to 200 Å) is formed through sputtering for 40seconds. The temperature at which, generally, GaN is epitaxially grownon a sapphire substrate in a c-axis direction (i.e., in a directionperpendicular to the main surface of the substrate) is higher than1,100° C. Therefore, when the thickness of the AlN film 12 is regulatedto 150 Å or less, and the GaN growth temperature is regulated to 1,100°C. or lower, crystal growth can be controlled so that GaN crystal 13 isnot grown on the surface 10 a of the sapphire substrate 10 or the bottomsurfaces 11 b of the grooves 11, and that growth of GaN crystal 13 onthe side surfaces 11 a of the grooves 11 proceeds predominantly in ac-axis direction.

When GaN crystal 13 is grown as described above, growth of GaN crystal13 proceeds rapidly toward the inside of each groove 11 in a c-axisdirection (-c direction); i.e., in a direction horizontal to thesapphire substrate 10, and also proceeds gradually in a directionperpendicular to the sapphire substrate 10 (FIG. 5C). When crystalgrowth proceeds further, the grooves 11 are filled with GaN, and thesurface 10 a of the sapphire substrate 10 is gradually covered with GaNthrough crystal growth in directions horizontal to the sapphiresubstrate 10 (i.e., both -c direction and +c direction). Finally, theflat GaN crystal 13 is formed on the sapphire substrate 10 (FIG. 5D).The thus-formed GaN crystal 13 has an m-plane main surface, since theside surfaces 11 a of the grooves 11 of the sapphire substrate 10 assumea c-plane. This is attributed to, for example, the difference in latticeconstant between GaN and sapphire.

As described above, the GaN template substrate production method ofEmbodiment 2 produces GaN crystal 13 having an m-plane main surface andexhibiting good crystallinity and surface flatness. Conceivable reasonstherefor will be described below. The present inventors executed ICPetching of an a-plane main surface of a sapphire substrate withoutgrooves; formed an AlN film through sputtering without restoring damageto the sapphire substrate caused by ICP etching; and grew a GaN crystalon the AlN film. As a result, a flat GaN crystal having a c-plane mainsurface failed to be formed on the sapphire substrate having an a-planemain surface. Therefore, conceivably, damage to the sapphire substratecaused by ICP etching prevents growth of a GaN crystal having a c-planemain surface on the sapphire substrate having an a-plane main surface.Also, conceivably, when the thickness of the AlN film 12 or the growthtemperature of GaN crystal 13 is regulated to an appropriate value, GaNcrystal 13 is not grown on the surface 10 a of the sapphire substrate 10or the bottom surfaces 11 b of the grooves 11 (i.e., GaN crystal 13 isepitaxially grown only on the side surfaces 11 a of the grooves 11), andgrowth of GaN crystal 13 proceeds predominantly in a c-axis direction;i.e., in a direction horizontal to the sapphire substrate 10. In thiscase, the surface of the GaN crystal 13 parallel to the sapphiresubstrate 10 assumes an m-plane, which is attributed to, for example,lattice matching between sapphire and GaN. For these two reasons, theGaN crystal 13 does not contain a crystal having a c-plane surfaceparallel to the main surface of the sapphire substrate 10, and has anm-plane main surface. Since growth of GaN crystal 13 proceedspredominantly in a direction horizontal to the sapphire substrate 10,the GaN crystal 13 can rapidly cover the surface 10 a of the sapphiresubstrate 10, and the surface 13 a of the GaN crystal 13 becomes flat.

Dependency of the crystallinity of GaN crystal 13 and the flatness ofthe surface 13 a was evaluated by the following experiments.

FIG. 6 shows the results of X-ray diffractometry of a surface 13 a of aGaN crystal 13 grown at 1,040° C. on a sapphire substrate 10 having(diameter: 2 inches) having grooves 11 (width: 1.5 μm, depth: 0.7 μm,interval: 1.5 μm). The time of sputtering for formation of an AlN film12 was varied from 5 to 30 seconds at intervals of 5 seconds. As shownin FIG. 6, when the sputtering time for formation of the AlN film 12 was5, 10, 15, 20, 25, or 30 seconds, peaks attributed to (10-10) plane and(20-20) plane (both assume an m-plane) were observed, and the thus-grownGaN crystal 13 was found to have an m-plane main surface. When thesputtering time was 5, 10, or 15 seconds, a peak attributed to (0002)plane (c-plane) was observed, and the thus-grown GaN crystal 13 wasfound to contain a crystal having a c-plane main surface. The intensityof the peak attributed to (0002) plane was gradually reduced as thesputtering time increased from 5 seconds to 15 seconds, and the peakattributed to (0002) plane was not observed when the sputtering time was20 to 30 seconds. A peak attributed to (0004) plane (c-plane) wasobserved only when the sputtering time was 5 seconds; i.e., this peakwas not observed when the sputtering time was 10 to 30 seconds. No peakattributed to a-plane was observed at any of the aforementionedsputtering times.

Thus, when the sputtering time was increased from 5 seconds to 15seconds, there was produced a GaN crystal containing a small amount of acrystal having a c-plane main surface, and exhibiting goodcrystallinity. When the sputtering time was 20 to 30 seconds, there wasproduced a GaN crystal containing virtually no crystal having a c-planemain surface, and exhibiting the best crystallinity. When the sputteringtime is 40 seconds, the thickness of the resultant AlN film 12 is about150 to about 200 Å. Therefore, when the sputtering time is 20 to 30seconds, the thickness of the resultant AlN film 12 is considered to beabout 75 to about 150 Å.

FIG. 7 is a graph showing the relationship between sputtering time forformation of an AlN film 12 and X-ray rocking curve half width in thec-plane or m-plane of a GaN crystal 13. The sizes of the sapphiresubstrate 10 and the grooves 11, and the growth temperature of GaNcrystal 13 were the same as those in the experiment whose results areshown in FIG. 6. The sputtering time for formation of the AlN film 12was varied in the same manner as in the case of the experiment whoseresults are shown in FIG. 6 (i.e., varied from 5 to 30 seconds atintervals of 5 seconds). For obtaining data corresponding to m-plane,the X-ray rocking curve half widths in c-axis and a-axis directions ofm-plane of a sample were determined by rotating the sample about boththe a-axis and the c-axis. In the plotted data of X-ray rocking curvehalf width shown in FIG. 7, squares, triangles, and circles correspondto c-plane, c-axis direction of m-plane, and a-axis direction ofm-plane, respectively.

As shown in FIG. 7, the X-ray rocking curve half width in a c-axisdirection of m-plane was 600 to 800 arcsec when sputtering time was 10to 25 seconds, and was approximately doubled when sputtering time was 5seconds or 30 seconds. Thus, the crystal orientation in a c-axisdirection of m-plane was high when sputtering time was 10 to 25 seconds.The X-ray rocking curve half width in an a-axis direction of m-plane was400 to 600 arcsec at any of the aforementioned sputtering times; i.e.,the crystal orientation in an a-axis direction of m-plane was high,regardless of sputtering time. The X-ray rocking curve half width in thec-plane was about 1,000 arcsec (sputtering time: 20 seconds or 25seconds), about 1,200 arcsec (sputtering time: 10 seconds, 15 seconds,or 30 seconds), and about 1,500 arcsec (sputtering time: 5 seconds).Thus, when the sputtering time was 20 to 25 seconds, the crystalorientation in the c-plane was higher, as compared with the case ofanother sputtering time.

As is clear from the data shown in FIGS. 6 and 7, the GaN crystal 13exhibits the most excellent crystallinity when the sputtering time is 20to 25 seconds. In this case, the thickness of the AlN film 12 isconsidered to be about 75 to about 125 Å.

FIG. 8 shows photographs of the surfaces of GaN crystals 13corresponding to the cases where the sputtering time for formation of anAlN film 12 was varied from 5 to 30 seconds at intervals of 5 seconds.The sizes of the sapphire substrate 10 and the grooves 11, and thegrowth temperature of GaN crystal 13 were the same as those in theexperiment whose results are shown in FIG. 6 or 7. As is clear from FIG.8, when the sputtering time is 15 seconds to 25 seconds (in particular,25 seconds), high surface flatness is achieved, whereas the sputteringtime is 10 seconds or 30 seconds, surface flatness is slightly impaired.As is also clear from FIG. 8, when the sputtering time is 5 seconds,surface flatness is lower as compared with the case where the sputteringtime is 15 seconds to 25 seconds, but higher as compared with the casewhere the sputtering time is 10 seconds or 30 seconds. Conceivably, whenthe sputtering time is 15 seconds to 25 seconds, the thickness of theAlN film 12 is 55 to 125 Å, whereas when the sputtering time is 25seconds, the thickness of the AlN film 12 is 90 to 125 Å.

GaN crystals 13 were grown at different growth temperatures, and thesurfaces 13 a of the thus-grown GaN crystals 13 were observed. The GaNcrystals grown at 1,020 to 1,060° C. exhibited high surface flatness.Particularly, the GaN crystals grown at 1,030 to 1,050° C. exhibitedhigher surface flatness. The GaN crystal grown at 1,040° C. exhibitedthe most excellent surface flatness.

When, in the heating step, heating to a growth temperature was carriedout in an atmosphere containing hydrogen and ammonia, the hydrogencontent of the atmosphere was varied. The crystallinity of the resultantGaN crystal 13 or the flatness of the surface 13 a of the GaN crystal 13was slightly higher when the hydrogen content was increased.

FIG. 9 is a graph showing the relationship between sputtering time forformation of an AlN film 12 and X-ray rocking curve half width in thec-plane or m-plane of a GaN crystal 13. The diameter of the sapphiresubstrate 10 is 3 inches. The grooves 11, and the growth temperature ofGaN crystal 13 were the same as those in the experiment whose resultsare shown in FIG. 6. The sputtering time for formation of the AlN film12 was varied from 10 to 25 seconds at intervals of 5 seconds. Forobtaining data corresponding to m-plane, the X-ray rocking curve halfwidths in c-axis and a-axis directions of m-plane of a sample weredetermined by rotating the sample about both the a-axis and the c-axis.In the plotted data of X-ray rocking curve half width shown in FIG. 9,squares, triangles, and circles correspond to c-plane, c-axis directionof m-plane, and a-axis direction of m-plane, respectively.

As shown in FIG. 9, the X-ray rocking curve half width in a c-axisdirection of m-plane was 500 to 800 arcsec when sputtering time was 10to 20 seconds. Thus, the crystal orientation in a c-axis direction ofm-plane was high when sputtering time was 10 to 20 seconds. The X-rayrocking curve half width in an a-axis direction of m-plane was 400 to500 arcsec at any of the aforementioned sputtering times; i.e., thecrystal orientation in an a-axis direction of m-plane was high,regardless of sputtering time. The X-ray rocking curve half width in thec-plane was about 1,000 arcsec (sputtering time: 15 seconds), about1,300 arcsec (sputtering time: 20-25 seconds), and about 1,500 arcsec(sputtering time: 10 seconds).

According to FIGS. 7 and 9 it is understood that the present inventionis applicable to the large size diameter larger than 3 inches.

In Embodiment 2, a sapphire substrate is employed as the growthsubstrate. However, the growth substrate may be, instead of a sapphiresubstrate, a substrate formed of a hexagonal material such as SiC, Si,GaAs, ZnO, or spinel. In Embodiment 2, ICP etching is employed forforming grooves in the growth substrate. However, another dry etchingtechnique, or an etching technique other than dry etching may beemployed. In Embodiment 2, an AlN film is employed as the buffer film.However, the buffer film may be a film formed of, for example, GaN,AlGaN, AlInN, or AlGaInN. Particularly when the growth substrate is asapphire substrate, the Al compositional proportion of the material ofthe buffer film is preferably high (the material is most preferablyAlN), from the viewpoint of, for example, lattice matching. InEmbodiment 2, reactive magnetron sputtering is employed for formation ofthe buffer film. However, any other sputtering technique may beemployed.

Embodiment 2 corresponds to a method for producing a GaN templatesubstrate. However, the present invention is not limited to GaN, but isapplicable to a Group III nitride semiconductor such as AlN, AlGaN,InGaN, AlInN, or AlGaInN. The present invention is not limited to theproduction of such a Group III nitride semiconductor described inEmbodiment 2 (i.e., Group III nitride semiconductor having an m-planemain surface), and can produce a Group III nitride semiconductor whosemain surface is any non-polar plane or semi-polar plane, inconsideration of the crystal orientation of the main surface of thegrowth substrate employed, the crystal orientation of the side surfacesof grooves formed in the growth substrate, and the lattice constant ofthe growth substrate. For example, when a sapphire substrate having anm-plane main surface is employed, and grooves are formed so that theside surfaces thereof assume a c-plane, a Group III nitridesemiconductor having an a-plane main surface can be produced. Also, whena sapphire substrate having a c-plane main surface is employed, andgrooves are formed so that the side surfaces thereof are a-plane, aGroup III nitride semiconductor having an a-plane main surface can beproduced.

In Embodiment 2, grooves are formed so that they are arranged in astripe pattern as viewed from above. However, grooves may be formed inany pattern; for example, a grid pattern or a dot pattern. Since a GroupIII nitride semiconductor has polarity in a c-axis direction, when theside surfaces of a groove have different crystal orientations, a GaNcrystal having different polarity directions is grown. Therefore, whengrooves are formed in such a pattern that the side surfaces of eachgroove have many different crystal orientations, a Group III nitridesemiconductor crystal having many different polarity directions isproduced, which is not preferred. From such a viewpoint, stripe-patterngrooves are more advantageous than grooves formed in another pattern,since each stripe-pattern groove has only two side surfaces, and a GroupIII nitride semiconductor grown on the side surfaces exhibits polarityin only two directions. Stripe-pattern grooves are also advantageous inthat the side surfaces of each groove have a large area, and thus thecrystallinity and surface flatness of a Group III nitride semiconductorgrown on the side surfaces are higher than those of a Group III nitridesemiconductor grown on the side surfaces of grooves formed in anotherpattern.

In order to avoid growth of a crystal having different polaritydirections, some of side surfaces of grooves may be covered with, forexample, a mask, so that a Group III nitride semiconductor crystal isnot grown on the thus-covered side surfaces. For example, when groovesare formed in a stripe pattern, and one of two side surfaces of eachgroove is covered with a mask so that a Group III nitride semiconductoris grown only on the other side surface, the resultant Group III nitridesemiconductor exhibits polarity only in one direction; i.e., thesemiconductor exhibits good quality.

Alternatively, in order to avoid growth of a crystal having differentpolarity directions, side surfaces of grooves may be inclined so thatthe side surfaces exhibits a crystal orientation in which crystal growthis less likely to occur. When, for example, a sapphire substrate isemployed as a growth substrate, the smaller the angle between the sidesurfaces of grooves formed in the substrate and the c-plane or a-planeof sapphire, the more likely a crystal is grown on the side surfaces. Inthis case, when the side surfaces of the grooves assume a c-plane ora-plane, a crystal is most likely to grow on the side surfaces.Therefore, when, for example, side surfaces of grooves on which a GroupIII nitride semiconductor crystal is grown assume a c-plane, and theother groove side surfaces on which no crystal growth is desired areinclined with respect to c-plane, growth of a crystal having differentpolarity directions can be avoided.

According to the present invention, there is produced a Group IIInitride semiconductor whose main surface is a non-polar plane (e.g.,m-plane or a-plane) or a semi-polar plane (e.g., r-plane). Therefore,the present invention facilitates production of a Group III nitridesemiconductor device which is not affected by a piezoelectric field.

What is claimed is:
 1. A method for producing a Group III nitridesemiconductor product, said method comprising: forming a groove in amain surface of a growth substrate consisting of a sapphire throughetching; heating, in an atmosphere containing hydrogen and ammonia, thegrowth substrate comprising the formed groove to a temperature at whichthe Group III nitride semiconductor of interest is grown; andepitaxially growing the Group III nitride semiconductor selectively ononly surfaces of the sapphire which are exposed on side surfaces of thegroove of the growth substrate in a direction parallel to the mainsurface of the growth substrate at the growth temperature, wherein thegrowth temperature is regulated so that a growth rate in a directionperpendicular to the side surfaces of the groove is higher than a growthrate in a direction perpendicular to the main surface of the growthsubstrate.
 2. A method for producing a Group III nitride semiconductorproduct according to claim 1, wherein the growth temperature is lowerthan a temperature at which the Group III nitride semiconductor isepitaxially grown on a planar growth substrate uniformly in a directionperpendicular to the growth substrate.
 3. A method for producing a GroupIII nitride semiconductor product according to claim 1, wherein at leastone of the side surfaces of the groove assumes a c-plane of thesapphire, and the Group III nitride semiconductor is epitaxially grownon at least one of the side surfaces of the groove which assumes thec-plane of the sapphire.
 4. A method for producing a Group III nitridesemiconductor product according to claim 3, wherein the growth substratecomprises a sapphire substrate comprising an a-plane main surface.
 5. Amethod for producing a Group III nitride semiconductor product accordingto claim 3, wherein the growth substrate comprises a sapphire substratecomprising an m-plane main surface.
 6. A method for producing a GroupIII nitride semiconductor product according to claim 3, wherein thegroove is formed in a stripe pattern so that the groove comprises alongitudinal side surface which assumes the c-plane of the sapphire. 7.A method for producing a Group III nitride semiconductor productaccording to claim 1, wherein the growth temperature is in a range from1,020° C. to 1,100° C.
 8. A method for producing a Group III nitridesemiconductor product according to claim 1, wherein the etchingcomprises a dry etching.
 9. A method for producing a Group III nitridesemiconductor product according to claim 8, wherein the dry etchingcomprises an ICP etching.
 10. A method for producing a Group III nitridesemiconductor product according to claim 1, wherein at least one of theside surfaces of the groove comprises an a-plane of the sapphire.
 11. Amethod for producing a Group III nitride semiconductor product accordingto claim 10, wherein the Group III nitride semiconductor is epitaxiallygrown primarily on at least one of the side surfaces of the groove whichassumes the a-plane of the sapphire.
 12. A method for producing a GroupIII nitride semiconductor product according to claim 1, furthercomprising: between the forming the groove and the heating the growthsubstrate, forming a buffer film on the surface of the growth substratethrough sputtering.
 13. A method for producing a Group III nitridesemiconductor product according to claim 12, wherein a thickness of thebuffer film is regulated so that the Group III nitride semiconductor isgrown primarily on the side surfaces of the groove in a directionparallel to the main surface of the growth substrate.
 14. A method forproducing a Group III nitride semiconductor product according to claim1, wherein a growth direction of Group III nitride semiconductor grownin the direction perpendicular to the side surfaces of the grooveincludes a c-axis of the Group III nitride semiconductor.
 15. A methodfor producing a Group III nitride semiconductor product, said methodcomprising: forming a groove in a surface of a growth substrate throughetching; heating, in an atmosphere containing hydrogen and ammonia, thegrowth substrate comprising the formed groove to a temperature at whicha Group III nitride semiconductor of interest is grown; and epitaxiallygrowing the Group III nitride semiconductor on side surfaces of thegroove at the growth temperature, wherein the growth temperature isregulated so that the Group III nitride semiconductor is grown primarilyon the side surfaces of the groove in a direction parallel to a mainsurface of the growth substrate, wherein the growth substrate comprisesa sapphire substrate, and wherein at least one of the side surfaces ofthe groove comprises an a-plane of sapphire, and the Group III nitridesemiconductor is epitaxially grown primarily on at least one of the sidesurfaces of the groove which assumes the a-plane of the sapphire.
 16. Amethod for producing a Group III nitride semiconductor product accordingto claim 15, wherein the growth substrate comprises the sapphiresubstrate comprising a c-plane main surface.
 17. A method for producinga Group III nitride semiconductor product according to claim 15, whereinthe groove is formed in a stripe pattern so that the groove comprises alongitudinal side surface which comprises the a-plane of the sapphire.18. A method for producing a Group III nitride semiconductor product,said method comprising: forming a groove in a surface of a growthsubstrate through etching; heating, in an atmosphere containing hydrogenand ammonia, the growth substrate comprising the formed groove to atemperature at which a Group III nitride semiconductor of interest isgrown; epitaxially growing the Group III nitride semiconductor on sidesurfaces of the groove at the growth temperature, wherein the growthtemperature is regulated so that the Group III nitride semiconductor isgrown primarily on the side surfaces of the groove in a directionparallel to a main surface of the growth substrate; and between theforming the groove and the heating the growth substrate, forming abuffer film on the surface of the growth substrate through sputtering,wherein a thickness of the buffer film is regulated so that the GroupIII nitride semiconductor is grown primarily on the side surfaces of thegroove in a direction parallel to the main surface of the growthsubstrate.
 19. A method for producing a Group III nitride semiconductorproduct according to claim 18, wherein the growth substrate comprises asapphire substrate.
 20. A method for producing a Group III nitridesemiconductor product according to claim 19, wherein at least one of theside surfaces of the groove assumes a c-plane of sapphire, and the GroupIII nitride semiconductor is epitaxially grown primarily on at least oneof the side surfaces of the groove which assumes the c-plane of thesapphire.
 21. A method for producing a Group III nitride semiconductorproduct according to claim 20, wherein the growth substrate comprises asapphire substrate comprising an a-plane main surface.
 22. A method forproducing a Group III nitride semiconductor product according to claim20, wherein the growth substrate comprises a sapphire substratecomprising an m-plane main surface.
 23. A method for producing a GroupIII nitride semiconductor product according to claim 20, wherein thegroove is formed in a stripe pattern so that the groove comprises alongitudinal side surface which assumes the c-plane of the sapphire. 24.A method for producing a Group III nitride semiconductor productaccording to claim 20, wherein the buffer film comprises AlN, and thethickness of the buffer film is regulated to 150 Å or less.
 25. A methodfor producing a Group III nitride semiconductor product according toclaim 19, wherein at least one of the side surfaces of the groovecomprises an a-plane of sapphire, and the Group III nitridesemiconductor is epitaxially grown primarily on at least one of the sidesurfaces of the groove which assumes the a-plane of the sapphire.
 26. Amethod for producing a Group III nitride semiconductor product accordingto claim 25, wherein the growth substrate comprises a sapphire substratecomprising a c-plane main surface.
 27. A method for producing a GroupIII nitride semiconductor product according to claim 25, wherein thegroove is formed in a stripe pattern so that the groove comprises alongitudinal side surface which comprises the a-plane of the sapphire.28. A method for producing a Group III nitride semiconductor productaccording to claim 18, wherein the thickness of the buffer film isregulated to be smaller than that of a buffer film which is employed forepitaxially growing the Group III nitride semiconductor on a planargrowth substrate uniformly in a direction perpendicular to the growthsubstrate, and the growth temperature is regulated to be lower than atemperature at which the Group III nitride semiconductor is epitaxiallygrown on a planar growth substrate uniformly in a directionperpendicular to the growth substrate.
 29. A method for producing aGroup III nitride semiconductor product according to claim 18, whereinthe growth temperature is in a range from 1,020° C. to 1,100° C.
 30. Amethod for producing a Group III nitride semiconductor product accordingto claim 18, wherein the buffer film comprises Al_(x)Ga_(l-x)N (0≦x≦1).31. A method for producing a Group III nitride semiconductor productaccording to claim 30, wherein the buffer film comprises AlN.
 32. Amethod for producing a Group III nitride semiconductor product accordingto claim 18, wherein the etching comprises a dry etching.
 33. A methodfor producing a Group III nitride semiconductor product according toclaim 32, wherein the dry etching comprises an ICP etching.
 34. A methodfor producing a Group III nitride semiconductor product according toclaim 18, wherein the sputtering comprises a magnetron sputtering.